A typical optical communication system will include an optical source such as, for example, a light-emitting diode for converting an electrical input signal to an optical signal, a photodetector for converting the optical signal to an electrical output signal, and an optical fiber waveguide for guiding the optical signal from the optical source to the photodetector. In optical fiber communication systems, the maximum transmission distance is limited by fiber attenuation and fiber dispersion. When analog signals are used, the maximum transmission distance is further limited by optical source nonlinearities which introduce harmonic distortions and limit the amount of power that may be used without severe distortion of the transmitted analog signal. Optical source nonlinearities, however, are not a serious problem in the transmission of digital signals. Consequently, a great deal of attention has been focused on the development of digital transmission systems even though analog signals generally have a smaller signal bandwidth than digital signals. Since the bandwidth of optical fiber is large and the bandwidth of a digital voice signal is relatively small, a large number of digital voice channels may be transmitted through a single optical fiber. Even though the bandwidth of an analog voice channel is somewhat less than that of a digital voice channel, source nonlinearities do not allow as large a number of analog voice channels to be transmitted through a single fiber. For other applications, the difference between analog and digital bandwidth is much more dramatic. For example, the bandwidth of a digital video channel is typically 15 times larger than an analog video channel. Thus, it would be advantageous to transmit a voice or video and the like as analog rather than as digital signals if harmonic distortion could be significantly reduced or even eliminated.
Several compensation schemes to improve the linearity of the optical source and to reduce harmonic distortion are known. These schemes include complementary distortion, negative feedback, phase shift modulation, feedforward, and quasi-feedforward compensation. Complementary distortion cancels harmonic distortion by introducing additional distortion into the drive circuit of each source to compensate for the harmonic distortion. The problem with this scheme is that each drive circuit must be individually designed to match the distortion of the optical source. In the negative feedback scheme, a portion of the optical signal is used to provide a compensating feedback signal, but here transmission bandwidth, frequency range, and transmitted power are reduced. Phase shift modulation produces selective harmonic compensation of nonlinearities in pairs of optical sources having similar characteristics, but the available transmission bandwidth is again reduced. Feedforward compensation involves the generation of an error signal by comparing a portion of the main optical signal to the input signal. Responding to the error signal, a second optical source produces another optical signal which is combined with the main optical signal to generate a linearized output signal. As with the negative feedback scheme, the amount of transmitted power is reduced. Quasi-feedforward compensation schemes combine elements of the feedforward and complementary distortion schemes, but also suffer from a reduction in transmitted power. Each of these complex compensation schemes does reduce harmonic distortion in varying degrees, but most suffer from a reduction in transmission bandwidth, frequency range, or transmitted power.